Ceramic plate for side weir of twin drum type continuous casting apparatus

ABSTRACT

The present invention relates to a ceramic plate material for side dams of a twin-drum strip caster, the ceramic plate material containing Al of 9 mass % or more in terms of Al equivalent and having the properties of; bending strength at room temperature of not less than 120 MPa, bending strength at 1,000° C. of not less than 65 MPa, hardness (Hv) of 50 to 350, fracture toughness K IC  at 1,000° C. of not less than 1 MPa·m 1/2 , thermal conductivity in the temperature range from room temperature to 1,000° C. of not more than 8 W/(m·K), thermal shock resistance index R′ of not less than 800 W/m, and wettability with molten steel (contact angle θ) of not less than 120°: and allows molten stainless steel to be continuously cast for a long period of time.

TECHNICAL FIELD

The present invention relates to a ceramic plate material used for sidedams which interpose cooling drums from both sides and form a moltensteel pool in a twin-drum strip caster used for continuously casting acast steel while generating solidification shells on the peripheralsurfaces of a pair of cooling drums each of which has an axis parallelwith the other and rotates in inverse direction relative to the other.

BACKGROUND ART

In a twin-drum strip caster, basic properties such as heat resistance towithstand a molten steel temperature of about 1,600° C., thermal shockresistance to withstand a temperature difference of about 400° C., lessthermal deformation (less thermal expansion coefficient), lesswettability with molten steel (better detachability of a coagulation),and excellent wear resistance are required of a material used for sidedams which interpose cooling drums from the both sides and form a moltensteel pool (refer to Japanese Unexamined Patent Publication No.S62-166054).

At the present time, when longer service life of continuous castingtends to be required, in addition to the basic properties, excellenterosion resistance is also required of side dams, since they erode atthe portions where both the cooling drums and molten steel contact them(the portions of the molten steel side along the slide planes facing thecooling drums), and if the continuous casting goes on for an extendedperiod of time, the erosion increases and the service life of the sidedams shortens (refer to Japanese Unexamined Patent Publication No.H7-68354).

However, there has been to date no ceramic plate material availablewhich satisfies all of those requirements, and the plates have beenformed by sticking or laying together different ceramic materials eachof which satisfies some of the required properties at the portions whereprescribed properties are required (refer to Japanese Unexamined PatentPublication Nos. H3-207554, H7-60411, etc.).

Those plates demonstrate excellent performance as side dams in the caseof a short-time continuous casting operation, but are not adequate for along-time continuous casting operation, since the performance isinsufficient due to their structural restrictions.

As a ceramic plate material, boron nitride (BN) has mainly been used inspite of its high cost, since it has the advantages of satisfying manyof the above required properties (heat resistance, thermal shockresistance, less thermal deformation, less wettability with moltensteel, etc.), not causing cracks during actual casting, easily fittingwith the drum end faces at the initial stage because of its softness,thus forming negligible clearances between the side dams and the drumend faces, and thus preventing molten steel intrusion.

However, since BN is soft, it therefore has the disadvantages in that itis abraded by the cooling drums in a short period of time and cannotwithstanding a long-time continuous casting operation.

Therefore, to make up for the disadvantages of BN, some ceramic platematerials comprehensively having the properties required of side damshave been developed by combining other ceramics, for example, siliconnitride (Si₃N₄) and aluminum nitride (AlN), with BN.

As an example, the above-mentioned Japanese Unexamined PatentPublication No. H7-60441 discloses a ceramic plate material consistingof BN of 30 to 50 wt %, Si₃N₄ of 30 to 65 wt % and AlN of 5 to 15 wt %.

This plate material is a material wherein, to improve the wearresistance of BN, Si₃N₄, which has the advantages of withstanding along-time casting because of excellent wear resistance and small thermalexpansion and also having a low cost, and further AlN, which does nothave very excellent thermal shock resistance but has good wearresistance and excellent erosion resistance, are combined with BN at therate of 30 to 65 wt % and 5 to 15 wt %, respectively. Therefore, theplate material has moderate thermal shock resistance, wear resistance(it is preferable that the wear resistance of the plate material bemoderately lower than the wear resistance of the drums) and erosionresistance, and thus demonstrates the performance desirable for sidedams.

Further, Japanese Unexamined Patent Publication No. H7-68354 discloses aceramic plate material consisting of BN of 20 to 30 wt %, Si₃N₄ of 55 to77 wt % and AlN of 3 to 15 wt %. This plate material also contains AlNof 3 to 15 wt % and thus has excellent erosion resistance.

However, when side dams made out of a conventional ceramic platematerial are used in the continuous casting of molten stainless steel,the erosion by the chemical reaction of alloy components in the moltensteel and the mechanical erosion by the molten steel in the state ofsemi-solidification coact, and therefore the side dams erode radicallyat the portions contacting the portions where the cooling drums and themolten stainless steel contact with each other. As this kink of theerosion looks like line, the erosion may be defined as linear-wear. Ifthe erosion of the side dams is excessive as stated above, the side damshave to be changed frequently and the operation efficiency of the casterdeteriorates accordingly.

Further, since the side dams used for the continuous casting of moltenstainless steel also play the role of supporting molten steel containingseveral kinds of alloy components in large amounts and having a largespecific gravity at both sides of the cooling drums for a long time, theside dams are required to be superior than the conventional ceramicplate materials not only in erosion resistance but also in basicproperties.

DISCLOSURE OF THE INVENTION

Based on the above requirements, the object of the present invention isto provide a ceramic plate material for side dams having excellentproperties sufficient for the ceramic plate material to be usedcontinuously even in the case of continuously casting molten stainlesssteel for a long time.

A ceramic plate material for side dams used for the continuous castingof molten stainless steel is required to have excellent basic propertiesas well as erosion resistance, as stated above.

In particular, when the side dams are sometimes cooled at the portionscontacting the cooling drums, the cooling induces the formation anddeposition of skull, and therefore the stable operation of continuouscasting is hindered.

To cope with this, the present inventors focused attention on thethermal conductivity of a ceramic plate material and adopted it as anindex in order to realize the excellent basic properties of the presentinvention.

Though wettability with molten steel has long been one of the necessarybasic properties, there has been no ceramic plate material which hasbeen evaluated and specified with any sort of concrete index in terms ofdegree of wettability.

In the continuous casting of molten stainless steel in particular, skullformed and deposited on side dams falls off and induces a phenomenonknown as hot band (one of falling skull being caught between coolingdrums and expanding the gap between them, and as a result, the caststeel being red-heated in the shape of a band along the width direction)which hinders operation, and therefore it is necessary to suppress theformation and deposition of the skull to the utmost.

Therefore, the wettability of a ceramic plate material for side damswith molten steel is an important property for the sake of operationefficiency and also the maintenance and control of product quality.

In the present invention, the present inventors focused attention on thewettability with molten steel which was not evaluated before althoughthe wettability is an important index, and adopted it as an index forevaluating and realizing the excellent basic properties of the presentinvention together with other indexes.

The present invention is a ceramic plate material composed mainly of BN,Si₃N₄ and AlN, and in particular containing AlN, which is good for wearresistance and excellent in erosion resistance, in larger amount thanthat of conventional ceramic plate materials (5 to 15 wt %) in order toenhance erosion resistance.

The present inventors varied the content of AlN which enhances erosionresistance in a ceramic plate material composed mainly of BN, Si₃N₄ andAlN, and measured the linear-wear-proof index (an index showing thedegree of erosion resistance). The results are shown in FIG. 1. As canbe seen from FIG. 1, the present inventors found that in order to ensurethe erosion resistance required of the side dams used for the continuouscasting of molten stainless steel, it was necessary for a ceramic platematerial to contain Al of 9 mass % or more in terms of Al equivalent,more concretely AlN of more than 15 mass % to not more than 40 mass %.

Further, the present inventors, as a result of intensive research on thethermal conductivity and wettability with molten steel which wereadopted as indexes in the present invention, found that it was necessaryfor a ceramic plate material containing Al of 9 mass % or more in termsof Al equivalent to have a thermal conductivity of not more than 8W/(m·K) and wettability with molten steel (contact angle θ) of not lessthan 120° in order to suppress the formation and deposition of skull.

The present invention is based on the above findings and the gistthereof is as follows:

(1) A ceramic plate material for the side dams of a twin-drum stripcaster, the ceramic plate material containing Al of 9 mass % or more interms of Al equivalent, characterized by having the properties of:bending strength at room temperature of not less than 120 MPa, bendingstrength at 1,000° C. of not less than 65 MPa, hardness (Hv) of 50 to350, fracture toughness K_(IC) at 1,000° C. of not less than 1MPa·m^(1/2), thermal conductivity at a temperature from room temperatureto 1,000° C. of not more than 8 W/(m·K), thermal shock resistance indexR′ of rug not less than 800 w/m, and wettability with molten steel(contact angle θ) of not less than 120°.

(2) A ceramic plate material for the side dams of a twin-drum stripcaster according to the item (1), characterized by the Al content being12.5 mass % or more in terms of Al equivalent.

(3) A ceramic plate material for the side dams of a twin-drum stripcaster according to the item (1) or (2), characterized by consisting of,in terms of mass %, BN of not less than 5% to not more than 20%, AlN ofmore than 15% to not more than 40% and Si₃N₄ of not less than 40% to notmore than 80%.

(4) A ceramic plate material for the side dams of a twin-drum stripcaster according to the item (3), characterized by containing, in termsof mass %, BN of not less than 10% to less than 20%.

(5) A ceramic plate material for the side dams of a twin-drum stripcaster according to the item (3) or (4), characterized by furthercontaining, in terms of mass %, one or more of: Al₂O₃ of not less than1% to not more than 15%, MgO of not less than 1% to not more than 15%,ZrO₂ of not less than 1% to not more than 30% and Y₂O₃ of not less than1% to not more than 15%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the Al content in termsof Al equivalent and the linear-wear-proof index in a ceramic platematerial composed mainly of BN, Si₃N₄ and AlN.

BEST MODE FOR CARRYING OUT THE INVENTION

A ceramic plate material for the side dams of a twin-drum strip casteraccording to the present invention (hereunder referred to as the“invented material”) is explained further.

The invented material is required to contain Al of not less than 9 mass% in terms of Al equivalent in order to ensure excellent erosionresistance.

When Al is present in a ceramic plate material, the Al reacts withoxygen [O] in molten steel and alumina precipitates on the surfaces ofthe plates in the form of a film. This alumina film ensures erosionresistance by acting as a protective film, and thus in order to obtainan alumina protective film sufficient to realize excellent erosionresistance, it is necessary for the ceramic plate material to contain Alof not less than 9 mass % in terms of Al equivalent. Therefore, thelower limit of the Al content in terms of Al equivalent is set at 9 mass%.

Another preferable Al content in terms of Al equivalent for realizingexcellent erosion resistance is not less than 12.5 mass %, and a furtherpreferable Al content is not less than 16 mass %.

A preferable Al compound used for supplying Al in the present inventionis AlN, and it is necessary for the AlN to be contained in an amountexceeding 15% in order to ensure Al of not less than 9 mass % in termsof Al equivalent. However, AlN does not very good thermal shockresistance, although it has excellent erosion resistance, and thereforeAlN degrades the thermal shock resistance of a ceramic plate materialwhen added abundantly. Therefore, the upper limit of the AlN content isset at 40%.

Further, another range of AlN content of the invented material desirablefor realizing excellent erosion and thermal shock resistance is frommore than 15% to 35%, more preferably from 17.5% to 27.5%.

The mechanical properties of the invented material are prescribed asfollows: bending strength at room temperature of not less than 120 MPa,bending strength at 1,000° C. of not less than 65 MPa, hardness (Hv) of50 to 350, and fracture toughness K_(IC) at 1,000° C. of not less than 1MPa·m^(1/2).

The above properties are required because the basic mechanicalproperties necessary in the case of a side dams cannot be ensured ifbending strength at room temperature is less than 120 MPa, bendingstrength at 1,000° C. is less than 65 MPa, and fracture toughness K_(IC)at 1,000° C. is less than 1 MPa·m^(1/2). More concretely, if the basicmechanical properties of a ceramic plate material cannot be ensured, theceramic material will crack or break during operation due to theresistance against sliding between the ceramic material and a drum andthe impact force caused by cast steel or a drum when hot band isgenerated, and therefore stable sealing of molten steel cannot bemaintained.

Further, if the hardness (Hv) is less than 50, wear proceeds extremelyrapidly exceeding an appropriate limit caused by sliding between theceramic material and a drum, and thus the service life of a side dam isshort.

On the other hand, if the hardness (Hv) exceeds 350, desired wear doesnot happen in contrast, conformability (adhesiveness) between a drum anda ceramic material cannot be ensured, and as a result, the sealing ofmolten steel becomes unstable.

Other preferable mechanical properties of an invented material are:bending strength at room temperature of not less than 150 MPa, bendingstrength at 1,000° C. of not less than 80 MPa, hardness (Hv) of 100 to200, fracture toughness K_(IC) at 1,000° C. of not less than 1.5MPa·m^(1/2): and further preferable mechanical properties are; bendingstrength at room temperature of not less than 200 MPa, bending strengthat 1,000° C. of not less than 100 MPa, hardness (Hv) of 130 to 170,fracture toughness K_(IC) at 1,000° C. of not less than 2 MPa·m^(1/2).

The thermal properties of the invented material are prescribed asfollows: thermal conductivity of not more than 8 W/(m·K), and thermalshock resistance index R′ (=S(1−ν)λ/Eα) of not less than 800 W/m.

Here, the thermal shock resistance index R′ is an index which showswhether the thermal shock resistance is good or bad, and at least 800W/m is required of the invented material in order to realize excellentthermal shock resistance.

In thermal shock resistance index R′ (=S(1−ν)λ/Eα), S representsfracture strength, ν Poisson's ratio, λ thermal conductivity, E Young'smodulus, and α thermal expansion coefficient.

It is a feature of an invented material to focus attention or thethermal conductivity and specify its proper range, in addition to thethermal shock resistance index, as the thermal properties of theinvented material.

With regard to a ceramic plate material, even if its thermal shockresistance is good, if its thermal conductivity is high, the portionwhere the ceramic plate material directly contacts a cooling drum iscooled by the cooling drum and skull forms and deposits, even grow intohuge one sometimes at said portion. If skull sticking to a side damsfalls off accidentally, a hot band (a red-heated band appearing alongthe width direction of cast steel) will frequently appear on cast steelas explained above, and thus preventing a stable continuous castingoperation.

The present inventors had an idea that the thermal conductivity of aceramic plate material should be lower in order to make the long-lastingand stable continuous casting operation possible, and intensivelyinvestigated a proper range of the thermal conductivity.

As a result, it was confirmed that long lasting and stable continuouscasting operation could be maintained without the formation anddeposition of skull by using a ceramic plate material having a thermalconductivity of not more than 8 W/(m·K).

Therefore, with regard to the thermal properties of the inventedmaterial, not only is thermal shock resistance index R′ specified to benot less than 800 W/m, but thermal conductivity is also specified to benot more than 8 W/(m·K).

Further, other preferable thermal properties of the invented materialare a thermal conductivity of not more than 6 W/(m·K), and a thermalshock resistance index R′ of not less than 1,200 w/m; and morepreferable thermal properties thereof are a thermal conductivity of notmore than 4 W/(m·K), and a thermal shock resistance index R′ of not lessthan 1,500 W/m.

With regard to the chemical properties of the invented material, erosionresistance is maintained at a high level by controlling the Al contentto be not less than 9 mass % in terms of Al equivalent, and wettabilitywith molten steel (contact angle θ) is specified to be not less than120°.

Basically, the lower the wettability with molten steel, the better theperformance of the material. Though this has been known from the past,the concrete proper range thereof has not been regarded as an object ofinvestigation and research.

The present inventors, as a result of intensive investigation into theproper range, found that it was necessary to maintain the wettabilitywith molten steel (contact angle θ) at not less than 120° for furthersuppressing the formation and deposition of skull on a side dam andensuring a long-lasting and stable continuous casting operation.

In this investigation, casting was conducted using various kinds ofceramic materials whose wetting angle with SUS304 molten steel wasmeasured with a high temperature microscope as actual plate materials,and the amount and size of hot band (the adhesiveness of skull) and thedegree of molten steel intrusion at a portion where molten steel wassealed were evaluated As a result, it was found that a ceramic materialhaving a contact angle of not less than 120° could be stably used as aplate material.

For that reason, the lower limit of the proper range of wettability withmolten steel (contact angle θ) is set at 120° for the invented material.

Another preferable wettability (contact angle θ) of the inventedmaterial is not less than 130°, and more preferable wettability (contactangle θ) is not less than 150°.

The invented material is characterized by containing, as its components,in terms of mass %, BN of not less than 5% to not more than 20%, AlN ofmore than 15% to not more than 40%, and Si₃N₄ of not less than 40% tonot more than 80%, in order to ensure the desired mechanical, thermaland chemical properties.

Si₃N₄ is used as one of the main components, but if its content is lessthan 40%, the required strength, hardness, wear resistance, etc. cannotbe obtained, and therefore the lower limit is set at 40%.

On the other hand, if Si₃N₄ is added in excess of 80%, the properbalance between wear resistance and fracture toughness cannot beobtained, and therefore the upper limit is set at 80%.

Another preferable range of Si₃N₄ content is 50 to 70%, and morepreferable range thereof is 55 to 65%.

BN is a ceramic material which has been used in the past, and it iscontained in the invented material within a range whose upper limit is20%. This is due to the fact that if it is contained in excess of 20%,the level of wear resistance required of the invented material cannot beobtained, though desired thermal properties can be obtained.

However, if its content is less than 5%, the level of thermal propertiesand wettability required of the invented material cannot be obtained,and therefore the lower limit is set at 5%.

Another preferable range of BN is 10 to less than 20%, and morepreferable range thereof is 12.5 to 17.5%.

The range of AlN is the same as stated above.

The invented material contains BN, AlN and Si₃N₄ as its main components,and also may contain one or more kinds of other ceramic materials withina range not detrimental to the required properties of the inventedmaterial; such as Al₂O₃ of not less than 2% to not more than 15%, MgO ofnot less than 1% to not more than 15%, ZrO₂ of not less than 1% to notmore than 30%, and Y₂O₃ of not less than 1% to not more than 15%.

EXAMPLE

Side dams were constructed using a ceramic material (BN: 15% and Si₃N₄:60%) containing Al of 15 mass % in terms of Al equivalent and having theproperties of: bending strength at room temperature of 180 MPa, bendingstrength at 1,000° C. of 100 MPa, hardness (Hv) of 150, fracturetoughness K_(IC) at 1,000° C. of 1.5 MPa·m^(1/2), thermal conductivityin the temperature range from room temperature to 1,000° C. of 6W/(m·K), thermal shock resistance index R′ of 1,300 W/m, and wettabilitywith molten steel (contact angle θ) of 130° C., and continuous castingof molten stainless steel was conducted. As a result, ten consecutive ofsix heat continuous casting operations (total steel weight: 360 tons)could be performed.

COMPARATIVE EXAMPLE

Side dams were constructed using a ceramic plate material not conformingto the conditions specified in the present invention, and continuouscasting of molten stainless steel was conducted. The results are set outbelow.

(1) In the case of side dams using a ceramic material containing Al of 5mass % in terms of Al equivalent and comprising BN of 20% and Si₃N₄ of70%, the erosion of the portions of the ceramic material which contactthe portions where cooling drums and molten steel contact with eachother was excessive, and the sealing of the molten steel was compromisedand thus the operation was stopped when the amount of cast steel reached100 tons.

(2) In the case of side dams using a ceramic material having theproperties of; bending strength at room temperature of 80 MPa, bendingstrength at 1,000° C. of 40 MPa, and fracture toughness K_(IC) at 1,000°C. of 0.8 MPa·m^(1/2), the ceramic material broke off at the lowestportion, and the sealing of the molten steel was compromised and thusthe operation was stopped when the amount of cast steel reached 100tons.

(3) In the case of side dams using a ceramic material having thermalconductivity in the temperature range from room temperature to 1,000° C.of 15 W/(m·K), the amount of skull deposited on ceramic dams duringoperation was large and hot band frequently occurred, and when theamount of cast steel reached 90 tons, a large hot band was generated andthe cast steel broke.

(4) In the case of side dams using a ceramic material having wettabilitywith molten steel (contact angle θ) of 90°, the amount of skulldeposited on ceramic dams was large from immediately after start of theoperation and hot band frequently occurred, and since the quality ofcast steel could not be maintained at a desired level, the operation wasstopped when the amount of cast steel reached 120 tons.

(5) In the case of side dams using a ceramic material having a hardness(Hv) of 40, wear of the w ceramic material proceeded quickly duringoperation, and when the amount of cast steel reached 60 tons, theoperation was stopped, since the amount of wear had reached theallowable limit.

(6) In the case of side dams using a ceramic material having a hardness(Hv) of 400, the ceramic material did not wear from the initial stage ofthe operation, the sliding conformance between the side dams and thecooling drums remained inferior, molten steel could not be sealedstably, and as a result, the operation was stopped when the amount ofcast steel reached 30 tons.

(7) In the case of side dams using a ceramic material having a thermalshock resistance index R′ of 500 W/m, the ceramic material broke off atthe initial stage of the operation, and as a result, the operation wasstopped when the amount of cast steel reached 15 tons.

INDUSTRIAL APPLICABILITY

According to the present invention, even though the amount of castingper one operation increases in the continuous casting of moltenstainless steel, the operation can be carried out stably andcontinuously for a long period of time, and therefore cast steel havinga uniform composition and structure can be obtained with high productionefficiency.

Therefore, the present invention greatly contributes to the developmentof continuous casting technology in the field of stainless steel.

1. A ceramic plate material for the side dams of a twin-drum stripcaster, the ceramic plate material consisting of, in terms of mass %, BNof not less than 5% to not more than 20%, AlN of more than 15% to notmore than 40% and Si₃N₄ of not less than 40% to not more than 80%, Al of9 mass % or more in terms of Al equivalent, and having the propertiesof: bending strength at room temperature of not less than 120 MPa,bending strength at 1,000° C. of not less than 65 MPa, hardness (Hv) of50 to 350, fracture toughness K_(IC) at 1,000° C. of not less than 1MPa·m^(1/2), thermal conductivity at a temperature from room temperatureto 1,000° C. of not more than 8 W/(m·K), thermal shock resistance indexR′ of not less than 800 W/m, and wettability with molten steel (contactangle θ) of not less than 120°.
 2. A ceramic plate material for the sidedams of a twin-drum strip caster according to claim 1 characterized bycontaining, in terms of mass %, BN of not less than 10% to less than20%.
 3. A ceramic plate material for the side dams of a twin-drum stripcaster, the ceramic plate material consisting of, in terms of mass %, BNof not less than 5% to not more than 20%, AlN of more than 15% to notmore than 40% and S₃N₄ of not less than 40% to not more than 80%, Al of9 mass % or more in terms of Al equivalent, one or more of: Al₂O₃ of notless than 1% to not more than 15%, MgO of not less than 1% to not morethan 15%, ZrO₂ of not less than 1% to not more than 30% and Y₂O₃ of notless than 1% to not more than 15%, and having the properties of: bendingstrength at room temperature of not less than 120 MPa, bending strengthat 1,000° C. of not less than 65 MPa, hardness (Hv) of 50 to 350,fracture toughness K_(IC) at 1,000° C. of not less than 1 MPa·m^(1/2),thermal conductivity at a temperature from room temperature to 1,000° C.of not more than 8 W/(m·K), thermal shock resistance index R′ of notless than 800 W/m, and wettability with molten steel (contact angle θ)of not less than 120°.
 4. A ceramic plate material for the side dams ofa twin-drum strip caster according to claim 3 characterized bycontaining, in terms of mass %, BN of not less than 10% to less than20%.